The present invention is related to railway car freight brake control valve devices and particularly to the quick service function provided by these control valve devices for effecting a local venting of the train brake pipe air at each car in order to propagate the brake pipe pressure reduction through the train when a brake application is initiated.
Quick service venting of brake pipe pressure is achieved, as a two-stage operation, in the present standard ABD and ABDW freight brake control valve devices. During the first stage, known as preliminary quick service, initial movement of the service piston slide valve from release position toward service position, in response to a train line reduction of brake pipe pressure, establishes a communication between the car brake pipe and a quick service bulb or volume that is, in turn, vented to atmosphere via a quick service exhaust choke. A local reduction of brake pipe pressure is effected by this communication to supplement the train line reduction of brake pipe pressure at each car, thereby serially transmitting the brake application signal from one car to the next car. In addition, this local reduction of brake pipe pressure encourages continued movement of the service piston to service position without hesitation, wherein the second stage of quick service is realized.
During this secondary stage of quick service, the service piston slide valve cuts off brake pipe pressure from the quick service bulb and connects the brake pipe pressure to the car brake cylinder device via the piston valve stem of the quick service limiting valve until approximately 10-12 psi brake cylinder pressure is developed. With the service piston in service position, it will be appreciated that auxiliary reservoir pressure is also connected to the car brake cylinder device concurrently with the brake pipe pressure during this secondary stage of quick service.
It has been found that, due to the relatively high pressure differential between brake pipe and auxiliary reservoir created by the first stage quick service reduction of brake pipe pressure, the degree of service piston movement into service position is such that maximum flow capacity is established at the service piston graduating valve port connecting auxiliary reservoir pressure to the brake cylinder device. The flow capacity of auxiliary reservoir pressure via this port opening momentarily exceeds the downstream flow capacity to the brake cylinder, thereby creating a short term back pressure surge or pressure pulse in the brake cylinder feedback passage to the quick service limiting valve device. Consequently, the brake cylinder pressure signal at the quick service limiting valve control piston reaches the cut-off value of 10-12 psi before the actual brake cylinder pressure builds up to this value. Accordingly, the quick service limiting valve is operated to interrupt the connection of brake pipe pressure to the brake cylinder device prematurely, and following dissipation of the back pressure pulse, re-establishes the brake pipe connection to the brake cylinder device.
Ideally, a smooth and continuous transition of the local quick service brake pipe pressure reduction should occur between the first stage preliminary quick service activity and the secondary stage of quick service. Due, at least in part, to this premature operation of the quick service limiting valve, however, a disruption occurs in the local quick service reduction of brake pipe pressure during the transition between the preliminary and secondary stages of quick service. This disruption is indicated by a time lag C, in the graph of FIG. 1, wherein curve A represents the secondary quick service reduction of brake pipe pressure achieved in accordance with the present invention and curve B represents the secondary quick service reduction of brake pipe pressure achieved in the standard ABD and ABDW control valve devices. As can be seen from this graph, during the secondary stage of quick service, curve B is displaced with respect to a curve D representing the brake pipe pressure reduction during preliminary quick service, so that within time period C, no local reduction of brake pipe pressure occurs.